1. Field of the Invention
This invention is in the field of continuous etching machines and is specifically directed to a control system for monitoring the etching capability of the etching solution to derive a signal which is an analog signal based upon etching solution strength. This signal may then be used for control purposes in adjusting the rate of material feed relative to the concentration of the etching solution.
2. Description of the Prior Art
During the etching of metal, for example, in the production of multi-layer wiring or etched molded components, the etching strength of the etching solution constantly decreases unless it is regenerated in a continuous process. Increased demands on the quality of the objects which are to be etched and the automation of etching processes necessitate that the period of dwell of the objects in the etching machine be precisely related to the etching strength of the etching solution.
There have been a number of processes and devices suggested for checking and controlling the etching depth in the prior art. For example, U.S. Pat. No. 3,503,817 describes a method for controlling the etching rate which involves the use of a sample test bath having a composition the same as that of the operating bath and in communication with the operating bath. In the sample test bath, a continuously moving test metal sample is continuously etched and the rate of dissolution is optically measured as a means for determining the relative strength of the etching solution.
The German Laid Open Application No. 1,521,706 describes a process in which the progress of the etching of objects passing through a spray etching device is determined at one or a plurality of measuring points by measuring the electrical conductivity of the objects, or measuring it optically, and the transport speed of the objects is appropriately adjusted.
U.S. Pat. No. 3,032,753 described an apparatus for controlling the depth of etching wherein metal strips having various thicknesses are submerged in the etching liquid. By means of two insulated electric supply lines, each metal strip is connected in series to a signal generator and a voltage source. During the etching process, the metal strips are etched through consecutively, starting with the thinnest. As soon as a metal strip has been etched through, its circuit is broken and a signal is emitted. The etching depth which has been reached can be determined from these signals.
The publication "IBM Technical Disclosure Bulletin" of June 1970, Vol. 13, No. 1, page 68, describes a process in which a check aperture is etched into the object to be etched during the etching process. The etching of the aperture is monitored by optical means with the aid of photocells and the time taken to etch through the aperture is used as a measure for the time required to complete the etching of the object.
German Utility Patent No. 71 47 646 describes a device for determining the etching strength of etching solutions in which a metallic wire is dipped into the etching solution at the exterior of the etching machine, and after etching, operates a time measuring device mechanically coupled to the wire to measure the interval of time between the submerging of the wire and the time of etching through.
German Laid Open Application No. 1,521,798 describes a process in which a conductive measuring wire is transported at a constant speed through the etching solution and the change in resistance or in the cross-section of the wire is constantly measured, and these results are used to monitor and control the process.
The monitoring and control means previously suggested have not been used extensively in practice because they have limitations in accuracy and require a considerable expenditure in terms of apparatus.